The principle of Doppler laser anemometry is to measure the frequency shift between the emitted laser beam and the beam backscattered by the particles (or aerosols) naturally present in the atmosphere, this frequency shift being directly proportional to the radial component (in other words along the laser line-of-sight) of the relative speed of the carrier with respect to the air. Hereinbelow, the speed along the laser line-of-sight is denoted more simply as the speed.
The method of measuring the speed of the aircraft conventionally comprises the following steps:                emission by the lidar of a laser beam,        reception by the lidar of the wave backscattered by particles present in the path of the emitted beam and generation, at the output of the photodetector, of the heterodyne signal resulting from the beat frequency between the backscattered wave and a fraction of the emitted wave, also referred to as local oscillator,        processing of the heterodyne signal by a processor unit installed onboard the aircraft in order to obtain the speed.        
Under certain atmospheric conditions, notably at high altitude, the atmospheric backscattering coefficient may, owing to the rarefaction of the aerosols, turn out to be insufficiently high to produce a usable LiDAR signal. Indeed, because of the quantum nature of light, the heterodyne signal is affected by a measurement noise interchangeably referred to as Schottky noise or photon noise to which may be added other contributors such as the intensity noise of the laser, the dark noise of the photodetector or again thermal noise. Furthermore, despite the amplification provided by the process of coherent detection, the Signal-to-Noise ratio remains potentially problematic. The single-particle mode of operation is appropriate for these situations. It consists in concentrating the laser power within a very small volume (˜0.1 mm3) by using a highly focused beam. When the “target” particles pass through the beam close to its focal point, they then produce an individually detectable signal.
The existing processing methods, one example of which is illustrated in FIG. 1a, consist in individually detecting, in the heterodyne signal, the useful signals produced by the passage of the particles in the laser beam in order to estimate their central frequency. The useful signals are analyzed using a time-frequency representation (TFR), well adapted to non-stationary signals, one example of which is illustrated in FIG. 1b. A useful signal is detected if its amplitude on the time-frequency representation is greater than a detection threshold defined as a function of the targeted maximum probability of false alarm. Then, the central frequency of each detected signal is estimated. The Doppler frequency on a line of sight is then estimated using the central frequencies of the single-particle signals detected over a given period of time.
In order to estimate the Doppler frequency—and as a consequence the speed—with a sufficient precision, it is necessary to maintain a low probability of false alarms which makes it essential to adopt a sufficiently high detection threshold. In order to obtain, for example, a precision having a standard deviation σ of around 0.2 MHz in Doppler frequency, which is equivalent to a precision having a standard deviation σ of around 0.15 m/s in speed, for a wavelength of λ˜1.5 μm, a threshold of around 12.5 to 13 dB above the mean level of the noise on the TFR must be adopted.
Such a processing method allows a satisfactory level of performance to be achieved in a large number of cases, but it does not offer a sufficient margin to guarantee the availability, the precision and the integrity of the measurement in any situation, as is required by the application areas such as aeronautics.
Consequently, there currently still remains a need for a method of measuring the speed of an aircraft that simultaneously gives satisfaction for all of the aforementioned demands, in terms of availability, of precision and of integrity, including in local and short-term situations of very low seeding of the atmosphere with particles.